In general, an electromagnetic steel sheet is used for an electric apparatus having an electromagnetic valve, a motor or a power supply circuit as a soft magnetic component. The soft magnetic component is required to have magnetic characteristics capable of acquiring a large magnetic flux density and capable of sensitively reacting against external field change.
When this soft magnetic component is used in an alternating magnetic field, energy loss referred to as iron loss takes place. This iron loss is expressed in the sum of hysteresis loss and eddy current loss. The hysteresis loss corresponds to energy necessary for changing the magnetic flux density of the soft magnetic component. The hysteresis loss, proportionate to the working frequency, is mainly dominant in a low-frequency domain of not more than 1 kHz. The term “eddy current loss” herein used denotes energy loss mainly resulting from eddy current flowing in the soft magnetic component. The eddy current loss, proportionate to the square of the working frequency, is mainly dominant in a high-frequency domain of at least 1 kHz.
The soft magnetic component is required to have a magnetic characteristic reducing this iron loss. In order to implement this, the permeability μ, the saturation magnetic flux density Bs and the electric resistivity ρ of the soft magnetic component must be increased, and the coercive force Hc of the soft magnetic component must be reduced.
In recent years, a powder magnetic core having smaller eddy current loss as compared with an electromagnetic steel sheet has attracted attention due to the progress of a high working frequency toward a high output and high efficiency of an apparatus. This powder magnetic core consists of a plurality of composite magnetic particles having metal magnetic particles and glassy insulating coatings covering the surfaces thereof. The metal magnetic particles are made of Fe, an Fe—Si-based alloy, an Fe—Al (aluminum)-based alloy, an Fe—N (nitrogen)-based alloy, an Fe—Ni (nickel)-based alloy, an Fe—C (carbon)-based alloy, an Fe—B (boron)-based alloy, an Fe—Co (cobalt)-based alloy, an Fe—P-based alloy, an Fe—P-based alloy, an Fe—Ni—Co-based alloy, an Fe—Cr (chromium)-based alloy or an Fe—Al—Si-based alloy.
In order to reduce the hysteresis loss in the iron loss of the powder magnetic core, the coercive force Hc of the powder magnetic core may be reduced by eliminating strains and dislocations from the metal magnetic particles and simplifying movement of magnetic walls. In order to sufficiently eliminate strains and dislocations from the metal magnetic particles, the molded powder magnetic core must be heat-treated at a high temperature of at least 400° C., preferably at a high temperature of at least 550° C., more preferably at a high temperature of at least 650° C.
However, the insulating coatings are made of an amorphous compound such as an iron phosphate compound, for example, due to requirement for resistance against powder deformation in molding, and attain no sufficient high-temperature stability. When an attempt is made to heat-treat the powder magnetic core at a high temperature of at least 400° C., the insulation properties are lost due to diffusion/penetration of the metallic elements constituting the metal magnetic particles into the amorphous substance. Thus, there has been such a problem that the electric resistivity ρ of the powder magnetic core is reduced to increase the eddy current loss when an attempt is made to reduce the hysteresis loss by high-temperature heat treatment. In particular, a small size, high efficiency and a large output have recently been required to the electric apparatus, and it is necessary to use the electric apparatus in a higher frequency domain in order to satisfy these requirements. Increased eddy current loss in the high-frequency domain hinders the attempt for attaining a small size, high efficiency and a large output of the electric apparatus.
In relation to this, Japanese Patent Laying-Open No. 2003-272911 (Patent Literature 1) or Japanese Patent Laying-Open No. 2003-303711 (Patent Literature 2), for example, discloses a technique capable of improving high-temperature stability of insulating coatings. The aforementioned Patent Literature 1 discloses a soft magnetic material of composite magnetic particles having insulating coatings of aluminum phosphate exhibiting high high-temperature stability. In the aforementioned Patent Literature 1, the soft magnetic material is manufactured in the following method: First, an insulating coating solution containing phosphate containing aluminum and dichromic salt containing potassium or the like, for example, is sprayed on iron powder. Then, the iron powder sprayed with the insulating coating solution is held at 300° C. for 30 minutes, and held at 100° C. for 60 minutes. Thus, insulating coatings formed on the iron powder are dried. Then, the iron powder formed with the insulating coatings is pressure-molded and heat-treated after the pressure molding, to complete the soft magnetic material.
The aforementioned Patent Literature 2 discloses iron-based powder, which is iron-based powder comprising powder, mainly composed of iron, whose surfaces are covered with coatings containing silicone resin and a pigment, and having coatings containing a phosphorus compound as underlayers of the coatings containing silicone resin and the pigment.    Patent Literature 1: Japanese Patent Laying-Open No. 2003-272911    Patent Literature 2: Japanese Patent Laying-Open No. 2003-303711